The present invention relates to apparatus and methods for determining the position of a piston within a cylinder and more particularly to the position of a piston within a propulsion cylinder of a propulsion system or tractor.
It is often important to know the position of a piston within a cylinder as the piston reciprocates within the cylinder. Typically the piston actuates another member such that it is important to know the position of the piston during its travel within the cylinder so as to know the position of the other member.
In particular, propulsion systems are used in the oilfield for propelling a bottom hole assembly within the borehole. Typically the propulsion system includes first and second housings with a packerfoot mounted on each of the housings with each housing having a piston and cylinder for moving the propulsion system within the borehole. The propulsion system operates by the packerfoot on one end of the first housing expanding into engagement with the wall of the borehole with the first piston then extending in the first cylinder to move the bottom hole assembly downhole. Simultaneously, the second packerfoot on the second housing contracts and moves to the other end of the second housing. Once the first piston completes its stroke, then the second piston in the second housing is actuated to propel the bottom hole assembly and propulsion system further downhole as the first packerfoot retracts and resets at the other end of the first housing. This cycle is repeated to continuously move the bottom hole assembly within the borehole. Thus, it is important to know the positioning of each of the pistons in the cylinders as the propulsion system cycles to know the position of the propulsion system as it propels the bottom hole assembly further downhole.
The sensor assembly includes a sensor mounted within the piston and a transmitter mounted either on the piston or cylinder. Preferably the transmitter includes a wheel disposed on the piston which engages the wall of the cylinder so as to rotate as the piston reciprocates within the cylinder. The wheel includes one or more magnets which emit a magnetic field that is received by the sensor as the wheel rotates. The sensor then sends an electric pulse signal to a logic component which determines the direction, travel, position, and speed of the piston within the cylinder.
More preferably, the wheel includes two oppositely charged magnets positioned on opposite sides of the wheel""s outer circumference. The sensor senses reversals in polarity of the two magnets every time the wheel rotates 180xc2x0. For every reversal in polarity, the sensor sends an electric pulse signal to the logic component. As the wheel rotates, the sensor transmits a stream of electric pulses as the wheel rotates. The direction, travel, position, and speed of the piston within the cylinder can then be determined by monitoring the number of pulses and the direction of piston travel. The position can be calculated from the wheel diameter, since each pulse corresponds to one-half of the wheel circumference.
Other objects and advantages of the invention will appear from the following description.